Prior to the present invention, as shown by Schwenker, U.S. Pat. No. 2,758,124, assigned to the same assignee as the present invention, there was provided a continuous process for the hydrolysis of organohalosilanes in which a two-phase aqueous organohalosilane mixture is pumped continuously through a loop to produce organopolysiloxane hydrolyzate and a 25-36 weight percent hydrochloric acid solution. The pump circulates the reaction mixture in the loop to effect intimate mixing of the reactants and circulation of the resulting organopolysiloxane hydrolyzate and acid. A heat exchanger is also used to achieve good heat transfer. The aqueous hydrochloric acid recovered from the hydrolysis mixture can be distilled to give anhydrous HCl or about a 21% hydrochloric acid solution which can be recycled. Although Schwenker's procedure results in the production of valuable silanol containing polydiorganosiloxane, as well as cyclopolydiorganosiloxane, the hydrolyzate often consists of polydiorganosiloxane having an average of more than 12 chemically combined diorganosiloxy units. The reactivity of such material is often unsuitable for making block copolymers or cyclopolydiorganosiloxane without further modification. In addition, Schwenker's procedure requires a mean residence time of at least 10 minutes for complete conversion of the chlorosilane.
Elaborate efforts have been made by those skilled in the art to minimize molecular weight build-up of organopolysiloxane hydrolyzate generated by chlorosilane hydrolysis. For example, isolation of freshly introduced chlorosilane from the hydrolysis reaction product was the basis of the invention of Gordon, U.S. Pat. No. 2,832,794. Gordon hydrolyzed chlorosilane with either water or aqueous hydrochloric acid which was introduced from a heat exchanger under a pressure of 10-30 psig. The aqueous stream was then passed through an eductor which effected the flow of controlled amounts of liquid chlorosilane. The dual component hydrolysis mixture was then conveyed into a mixing chamber which was followed by a packed column.
The present invention is based on the discovery that low molecular weight silanol terminated polydiorganosiloxane can be made readily by effecting contact in a static tubular reactor, organochlorosilane having the formula, EQU (R).sub.a SiX.sub.4-a, ( 1)
and water, or hydrochloric acid, which can be dilute, concentrated or saturated, where R is a C (1-13) monovalent organic radical, X is a halogen radical and a is an integer equal to 1 to 3 inclusive.
Complete conversion of the organochlorosilane of formula (1) to low molecular weight organopolysiloxane hydrolyzate can be achieved in less than about sixty seconds. The present invention also enables a three-phase flow of aqueous HCl, gaseous HCl and organopolysiloxane hydrolyzate in the form of low molecular weight silanol terminated polydiorganosiloxane and cyclopolydiorganosiloxane. The direct evolution of anhydrous HCl as a by-product of the reaction is also provided.
The process of the present invention can be practiced over a wide range of Reynolds Numbers (Re), which defines the degree of turbulence during hydrolysis, where Re is defined as EQU Re=(.rho.VD/.mu.)
.rho.=density of fluid PA1 V=fluid velocity PA1 D=diameter of static mixer and PA1 .mu.=viscosity of the fluid PA1 (1) introducing organochlorosilane and water, or hydrochloric acid in a proportion of from about 10 to 50 moles of water, per mole of organochlorosilane into a static tubular reactor having elements capable of directing material flow radially towards the reactor wall and back to the elements, where the resulting fluid material is conveyed through the reactor at a rate sufficient to achieve a degree of turbulent mixing capable of substantially converting the organochlorosilane to organopolysiloxane within 60 seconds or less, PA1 (2) recovering the organopolysiloxane from (1).
Depending upon the flow rate of the fluid mixture through the static mixer, an Re of less than 10 has been found effective, while a value of greater than 2000 is preferred.